The invention relates to one-bit analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). More particularly, the invention relates to such converters which employ an adaptive filter having two regimes or modes of operation.
One-bit digital systems have the great virtue that they do not require high precision components. The digital-to-analog converter (DAC) of a one-bit system consists simply of a low-pass filter 2 such as shown in FIG. 1a. The analog-to-digital converter (ADC), such as shown in FIG. 1b, generally consists of an identical filter 2, known as the local decoder, a comparator 4 comparing the analog input signal with the output of the local decoder, and a sampling system such as sampler and one-bit quantizer 6, typically implemented as a D flip-flop, clocked at regular intervals to deliver data pulses whose polarity (1 or 0) depends on whether the local decoder's output is positive or negative with respect to the input signal.
Because every ADC contains a DAC (a local decoder--usually just a filter), the following discussion is directed primarily to the filter itself. It will be understood that the filter is employed in an ADC or a DAC.
In the digital-to-analog converters of non-adapting one-bit systems, impulses are fed via a filter to yield the analog output signal. The impulses are of two amplitudes, corresponding to the 1 and 0 of a data bit (e.g., they might be +5 and -5 volts). The nature of the filter determines the type of one-bit system.
In Delta Modulation systems the filter is an integrator or a low-pass filter with a cut-off frequency below or near the bottom of the message band (the range of frequencies occupied by the signal to be conveyed), so that over that band the response of the filter falls progressively with increasing frequency, most commonly at 6 dB/octave.
In Delta-Sigma Modulation systems the filter is a low-pass filter with a cut-off frequency above or near the top of the message band, so that over that band the filter response is substantially flat. For example, in a system designed for high-quality audio where the message band might be 30 Hz to 15 kHz, a delta modulation system might use a single-pole filter with a cut-off at 100 Hz (often called a leaky integrator), while the filter of a delta-sigma modulation system might cut-off at 10 kHz.
One-bit systems may be adaptive. Presently known methods can be broadly divided into two types: amplitude variation and filter adaptation.
Amplitude variation is the method used in conventional adaptive delta modulation. The amplitude of the impulses is varied, either continuously or in discrete steps, before filtering. Control circuitry is designed so that increasing signal amplitude leads to increasing impulse amplitude. In the example of FIG. 2a, the size of the input pulses is modulated by a voltage-controlled amplifier (VCA) 8 which precedes a fixed low-pass filter 10, but other methods are possible. An alternative but equivalent configuration, shown in FIG. 2b, uses fixed amplitude impulses but follows the filter 10 by the VCA 8. Either approach yields a much improved signal-to-noise ratio for low-level signals, compared with a non-adapting system. The noise has a substantially constant spectrum but varies with signal level, being directly proportional to the pulse amplitude. This variation may be audible as noise modulation. It is particularly likely to be audible in the presence of high-frequency signals, which may mask the rise in noise at high frequencies but not the accompanying rise at low and middle frequencies.
The filter adaptation approach employs fixed amplitude pulses but varies the cut-off frequency of the filter, again either continuously or discretely. FIG. 3 shows such an arrangement having a variable low-pass filter 2'. Under no-signal or low-level conditions, the system is conventional delta modulation, generally with a filter frequency substantially below the bottom of the message band. As the signal level increases, the filter moves up but initially it still has a cut-off frequency below the message band. However, once the cut-off frequency is within the message band, the gain below the cut-off frequency becomes constant. Thus the system yields a noise spectrum which is variable, with much less low-frequency noise in the presence of high-frequency signals. At high signal levels the filter slides upwards in frequency to approach the top of the message band; the system changes from delta to delta-sigma modulation.
This technique of adapting the filter gives a better subjective signal-to-noise ratio in the presence of high-level high-frequency signals because under these conditions the low frequency gain of the filter is lower than that of an integrator, and therefore the noise spectrum contains less energy at low frequencies where there is little or no masking.
However, under no-signal conditions the filter is effectively a pure integrator over a wide range of frequencies from below to the top of the message band and beyond. Thus, compared with a system employing amplitude adaptation there is a greater amplification at very low frequencies and therefore a greater proportion of low frequency noise at the bottom of and below the audio spectrum.
This increased low-frequency noise may not of itself be a significant disadvantage since under these signal conditions the absolute filter gain in the audio band is small so the absolute noise level is low. In addition the human ear is very insensitive to low-level low-frequency sounds, so the extra low-frequency noise is unlikely to be audible.
However, the required voltage-controlled filter (VCF) will in practice employ some form of VCA. It is an undesirable property of most designs of VCA that a small proportion of the control signal is added to the controlled signal, so that even when trimmed to minimize the effect the output of the VCF may contain a voltage or current offset which varies with the position of the filter cut-off frequency. If the variable filter is effectively an integrator under small-signal conditions, this variable offset may be amplified excessively, yielding an audible "thump " on high-frequency transients and a visible displacement of the base-line when the output waveform is observed on an oscilloscope.
As explained above, compared with conventional (amplitude-) adaptive delta modulation, filter adaptation gives reduced audible noise modulation but demands better performance from the VCA used in the adapting circuit. In accordance with the present invention, an ADC or DAC employs an adapting filter which retains the advantage of the sliding low-pass filter under high-level conditions but does not demand less offset from the VCA under low-level conditions.
In addition, in accordance with the present invention, the advantage of the sliding band (variable filter cutoff frequency) is realized under high-level high-frequency conditions but without the excessive very low frequency gain under no-signal conditions, thus reducing very low-frequency noise in the quiescent or low-level state. Thus, ADCs and DACs in accordance with the invention provide the advantages of amplitude- and filter-adapting arrangements while avoiding their weak points.